1. Field of the Invention
This invention relates generally to magnetic field sensing devices, and more particularly, relates to a device for detecting displacement of a magnetoresistive element relative to a magnetic field.
2. Description of the Prior Art
Magnetic field sensors which provide an output voltage that varies in response to a changing magnetic field are widely used in instrumentation, both in precision measurement laboratory equipment and also in business equipment such as weighing scales, and also to perform such functions as controlling the rotation of a motor. Such magnetic sensor elements are also used in contactless switches and volume adjusting controls in audio equipment.
Such a magnetic sensor device can be constructed using a ferromagnetic metal element, a semiconductor magnetoresistive element, or a hall element. Heretofore, semiconductor magnetoresisitive elements and hall elements have been principally used in prior art magnetic sensor elements. However, semiconductor magnetoresistive elements and hall elements have characteristics which depend on the properties of the semiconductor material being used. For example, in magnetoresistive elements formed of semiconductor materials such as GaAs, InSb, or other magnetoresistive semiconductors, the charge carrier density and the ability of such charge carriers to move in the material is highly dependent upon temperature. Thus semiconductor magnetoresistive elements and hall elements have adverse temperature sensitivities, and have large variations in resistivity within the element, thereby requiring that an external temperature compensation circuit and a circuit to compensate for the dispersion resistivity be included. Furthermore, because the resistivity of the semiconductor magnetoresistive element varies approximately as the square of the intensity of the magnetic field when the latter is small, a relatively large magnetic field, such as one kilogauss, of bias magnetisim is usually required. Because the required flux density is so high the bias magnetic field cannot be provided with sufficient uniformity over a large region, and thus there will be lack of linearity of resistance throughout the element. As a result, it is extremely difficult for a magnetic sensor device using such a semiconductor magnetoresistive element to measure fine displacement with satisfactory linearity.
It is also known to use magnetoresistive elements formed of ferromagnetic material. Devices using such magnetoresistive elements to sense magnetic fields are described in U.S. Pat. Nos. 3,928,836; 3,949,345; 4,021,728; 4,053,829; and 4,021,728, each of which has been assigned to the assignee of the present application.
The magnetoresistive effect of ferromagnetic material, as used in such ferroresistive elements, can be explained by two separate phenomena.
The first phenomenon is the change in resistance which is produced through the change in self-magnetization caused by an outside magnetic field. This phenomenon can be explained by Mott's theory. In general, this first phenomenon results in the linear reduction of the resistivity of the device as the magnetic field is increased, and is isotropic with respect to the direction of the magnetic field. However, while this phenomenon is significant when the ferromagnetic material is heated to its curie temperature, where self-magnetization is most intense, but can be neglected so long as the external magnetic field is relatively small and the temperature is low.
The second phenomenon can be observed in a relatively small magnetic field, because the magnetoresistive effect in this phenomenon is anisotropic, that is, the resistivity of the ferromagnetic material depends on the angle between the direction of magnetization and the direction of current flow. This phenomenon is most intense in the temperature region where the change in self-magnetization is small, and grows smaller gradually as the temperature of the material is increased towards the curie temperature.
Generally, in a ferromagnetic metal, the resistivity is at a maximum when the directions of current and magnetization are parallel, and is at a minimum when those directions are perpendicular to one another. The phenomenon can be expressed in terms of the well-known Voight-Thomson formula; EQU .rho.(.theta.)=.rho..sub..perp. sin.sup.2 .theta.+.rho..sub..parallel. cos.sup.2 .theta. (1)
where .rho..sub..perp. and .rho..sub..parallel. are the resistivity of the ferromagnetic material when saturated with a magnetic field perpendicular to the direction of current flow, and parallel to the direction of current flow, respectively, and where .theta. is the angle between the direction of current flow and the direction of saturated magnetization. Ferromagnetic magnetoresistive elements utilizing this second phenonmenon are described in the aforementioned U.S. patents.